For each image, the left side is the a screenshot of the game running with ambient occlusion enabled, while the right side is a screenshot of the setting disabled. The right side produces a much cleaner looking image.

someone on gamedev.stackexchange.com wrote:Ambient light is a light type in computer graphics that is used to simulate global illumination. Ambient occlusion is simply a simulation of the shadowing caused by objects blocking the ambient light. Because ambient light is environmental, unlike other types of lighting, ambient occlusion does not depend on light direction. As such, it can be pre-computed for static objects.

The dirty, static effect might be shadows or just the occlusion's effect on the graphics, or maybe just a computation glitch.